This invention relates generally to variable optical attenuators of information networks. More particularly, embodiments of the present invention provide for a system and method for controlling a variable optical attenuator based on optical power at one or more downstream transceivers.
Optical networks are becoming widely used for distributing both high and low speed data over varying distances. Typically, an optical network is comprised of a number of network elements (NE) that are connected to each other in a variety of configurations so as to form a unified communication network. The communication network may extend over a small area, such as a company wide network, or may cover large distances, such as in regional or nationwide networks. Typically, the NE""s allow network clients to input data for transmission over the network and to receive data transmitted over the network from other locations. Thus, data may be added or dropped from the network at NE locations as the data flows from point to point throughout the network.
Typically, a network element in a wavelength division multiplexing (WDM) system includes one or more wavelength converters, that may be referred to as transceivers that convert optical signals from one wavelength to another. The transceivers are used as part of input or output tributaries associated with a network element For example, an input tributary allows a network user to input signals at an NE for transmission over the network, and an output tributary allows a network user to receive signals at an NE that have been received from the network.
FIG. 1 shows a typical WDM network element 100 that receives signals transmitted over a communication network and includes three transceivers that produce corresponding output signals to a local user. The network element couples to the communication network as shown at 102. The input signal from the network may be received by a line receiver 104. The line receiver may include at its output a variable optical attenuator (VOA) 106 that is used to adjust the power level of the received signal. For example, the VOA may be used to attenuate the received signal by a selectable attenuation factor. The output 108 of the line receiver 104 is coupled to a demulitplexer (demux) stage 110 that filters the received input signal to produce three demultiplexed signals (112, 114, 116) that are input to the three transceiver circuits 118, 120, 122, respectively. The transceiver circuits may convert their received signals to different optical wavelengths or to electrical signals for output as the respective output signals 124, 126, and 128.
It is very important to control the optical power input to the transceiver circuits (118, 120, 122) so that the transceivers operate optimally. If the power is too high, then the transceiver hardware can be damaged, and/or errors may creep into the signals. If the power is too low, then again the signals may not remain error-free. The network designer calculates the optimal receive power range for the transceiver inputs and the network installer (technician) needs to ensure that the optical received power at the transceivers is indeed in that range. For some traffic types, the optimal range is extremely narrow, such as no more than 2 dB, and so accurate transceiver input power settings are required.
In a typical optical network element, there may be one or more variable optical attenuators (VOAs) employed for controlling the magnitude of various optical powers of optical signals processed by the NE. For example, the line receiver 104 may include the VOA 106 that controls the optical power of the optical signal 108 that is input to the demux 110. Due to channel multiplexing included in the signal 102, the VOA 106 simultaneously controls the optical powers of the signals transmitted to the three transceivers. Thus, in some network element configurations, a VOA may operate to control the optical power ranges for multiple optical signals that are being transmitted to multiple downstream circuits.
A conventional VOA circuit includes a VOA controller (VOAC) 130 coupled to an optical PIN detector 132 to detect the power level at the output of the VOA 106. The optical PIN detector is a diode with a large intrinsic region sandwiched between p- and n-doped semiconducting regions. Photons absorbed in this region create electron-hole pairs that are then separated by an electric field, thus generating an electric current in a load circuit. The software operating on the VOAC implements a closed loop control of the optical output power of the VOA 106 by reading the PIN detector 132 and making adjustments to a VOA attenuation factor of the VOA 106 via control line 134, to achieve a desired optical output power from the VOA 106. The foregoing approach is problematic in the sense that while the desired intent is to control the optical power that is being received at the transceivers, the optical power that is actually being controlled is the power at the output of the VOA 106.
The software at the VOAC 130 controls optical output power to achieve an estimated received power at the transceivers by estimating the expected optical losses between the VOA output 108 and the transceivers. The software on the VOAC also assumes that the input power at the VOA 106 is comprised of channels at equal power levels. The estimated optical received power at the transceivers is typically wrong because the estimates of optical losses between the VOA output 108 and the transceivers are not accurate; and the power levels of the channels received at the VOA input may not be perfectly equalized. The software on the VOAC can only estimate the individual power of each channel at the output of the VOA through equal allocation of the total power among the channels. A wrong estimate can mean that the received optical power at one or more of the transceivers is outside the optimal range.
The VOA, however, can also be operated in an open-loop or incoming fashion where the VOA attenuation factor is set to a fixed level under manual control. In a manual control operation, a network operator or installer calculates or measures the received optical power at the transceivers (i.e., using an optical power meter), and after detection of the incoming optical power at the transceivers, the operator sets the fixed VOA attenuation factor in an attempt to achieve the optimal setting. A wrong adjustment by the operator could result in irreparable damage to one or more of the transceivers. Thus, a VOA operating in open-loop mode is unable to accommodate changes (e.g., due to aging or network upgrades) in the attenuation of the VOA, and may risk damaging expensive network components.
Therefore, what is needed and what has been invented is a system and method for adjusting optical signal power levels received at one or more transceivers without relying on an operator to monitor the optical power of optical signals, and accordingly adjust the VOA attenuation factor in an attempt to achieve a desired optical setting. The system and method of the present invention implements a distributed closed-loop operation of the VOA. Instead of using the optical power measured at the PIN detector 132 at the output of the VOA 106, the system and method of the present invention utilize optical power reading at one or more downstream transceivers to adjust the VOA attenuation settings.
The present invention provides a system and method for adjusting optical signal power levels in an optical network. The system comprises a variable optical attenuator (VOA) for receiving an input signal and producing an attenuated optical output signal. A VOA controller (VOAC) is coupled to the VOA for adjusting a VOA attenuation factor of the VOA from optical power level changes detected at downstream network components. For example, the power level at any downstream component or circuit card input can be detected and controlled in accordance with the present invention.
In one embodiment of the invention, the power level input to a transceiver circuit pack is controlled in accordance with the present invention. The transceiver circuit pack is optically coupled to receive an optical input signal derived from the attenuated optical output signal output from the VOA. The transceiver includes an optical power level detector optically communicating with the optical input signal for detecting the optical input signal and producing a detected optical power level value. The system for adjusting optical signal power also comprises a signaling channel coupled to the detector and the VOAC for transmitting the power level value from the detector to the VOAC. The VOAC receives the power level value and accordingly adjusting the VOA attenuation factor of the VOA to achieve a desired power level value at the input of the transceiver.
In one embodiment of the invention, a system is provided for adjusting a power level of an optical signal at an input to a component in a network element that forms part of an optical network. The system includes a variable optical attenuator (VOA) for receiving a first optical input signal and producing an attenuated optical output signal. The VOA includes a control input to receive control parameters that adjust a VOA attenuation factor. The system also includes a signaling channel and a detector coupled to the input of the component for detecting a power value of a second optical input signal that is derived from the attenuated optical output signal. The detector is further coupled to the signaling channel and includes logic to transmit the power value on the signaling channel. The system also includes a VOA controller (VOAC) coupled to the VOA and the signaling channel. The VOAC includes logic for receiving the power value from the signaling channel and generating selected control parameters that are input to the control input of the VOA for accordingly adjusting tho VOA attenuation factor to achieve a selected signal power level at the input to the component.
In one embodiment of the invention, a method is provided for adjusting a power level of an optical signal at an input to a component in a network element that forms part of an optical network. The method includes the steps of attenuating a first optical signal with a VOA to produce an attenuated optical signal, the VOA having a plurality of attenuation factors, transmitting a second optical signal that is derived from the attenuated optical signal to the input of the component, detecting a power value for the second optical signal at the input of the component, and selecting one of the VOA attenuation factors based on the power value so that a selected power value is detected at the input of the component.